1. Field of the Invention
The present invention relates to a semiconductor device which has a light source and is structured by photoelectric conversion elements arranged and a plurality of transistors (hereafter referred to as TFTs) in a matrix shape. Further, the present invention relates to a semiconductor device which has photoelectric conversion elements, light emitting elements, and a plurality of transistors formed on an insulating surface or on a semiconductor substrate. The semiconductor device of the present invention has a function as an image sensor and a display function of an image.
2. Description of the Related Art
Developments in solid state imaging devices which has photoelectric conversion elements such as diodes and CCDs, for reading out electric signals with image information, from character information, drawing information, or the like on a page, have been advancing in recent years. The solid state imaging devices are used in devices such as scanners and digital cameras.
Solid state imaging devices which has photoelectric conversion elements can be roughly categorized into line sensor and area sensor types. Line sensors scan a subject using photoelectric conversion elements formed in a linear shape to take in image information as an electric signal.
In area sensors, on the other hand, also referred to as contact type area sensors, photoelectric conversion elements provided in a plane (planar shape) are disposed on a subject and then take in image information as an electric signal. Unlike line sensors, it is not necessary to perform scanning operations of the photoelectric conversion elements with area sensors, and therefore it is unnecessary to employ components such as a motor used during scanning.
Devices which have image sensors such as line sensors or area sensors are referred to as semiconductor devices in this specification. FIG. 5 shows a schematic diagram of the structure of a conventional semiconductor device. Reference numeral 1001 denotes a CCD (CMOS) image sensor, and an optical system 1002 such as a rod lens array is disposed on the image sensor 1001. The optical system 1002 is disposed so that an image of a subject 1004 is reflected on (irradiated to) the image sensor 1001. In FIG. 5, the image relationship of the optical system 1002 is taken as being non-magnifying. A light source 1003 is disposed in a position where light can be irradiated to the subject 1004. Components such as LEDs or fluorescent lamps are employed as the light source 1003 used for the semiconductor device shown in FIG. 5. A glass 1005 is then placed below the subject 1004. The subject 1004 is disposed above the glass 1005.
Light emitted form the light source 1003 is irradiated to the subject 1004 through the glass 1005. The light irradiated to the subject is reflected by the subject 1004, and then made incident upon the optical system 1002 through the glass 1005. The light made incident to the optical system 1002 then inputs to the image sensor 1001, and information of the subject 1004 undergoes photoelectric conversion in the image sensor 1001. Then, a signal showing information on the subject 1004 which was electrically inverted is read out to the outside. The image sensor 1001 reads out information of the subject 1004 line by line, a scanner 1006 is moved after one line portion is read by the image sensor 1001, and then similar operations are repeated.
Light from the light source 1003 is irradiated to the subject 1004 through a medium, the glass 1005, in the semiconductor device described above and shown in FIG. 5. Thus, there are cases in which the light is not uniformly irradiated with this structure, and this becomes a problem. Further, the light reflected by the subject 1004 is irradiated to the image sensor 1001 through another medium, the optical system 1002. Therefore, There is occurred a problem that irregularities are caused by the fact that the image becomes lighter in certain portions and darker in other portions when the information subject 1004, which is read in, is shown on the image.
In addition, it is difficult to control the size of the optical light system 1002 and the size of the light source 1003 with the aforementioned structure of the semiconductor device. Namely, it is difficult to make the size of the optical system 1002 and the size of the light source 1003 smaller than a certain fixed size. As a result, the semiconductor device itself is prevented from being made smaller and thinner.
With the above circumstances, an object of the present invention is therefore to provide a semiconductor device in which irregularities in bright ness are not caused in a read-in image. In addition, an object of the present invention is to provide a semiconductor device that has been made smaller and thinner.
The present invention provides a semiconductor device in which a plurality of pixels are formed in matrix on the same substrate, each pixel has a photoelectric conversion element, a light emitting element, and a thin film transistor (TFT) for controlling the elements. A semiconductor device that has been made smaller and thinner can be provided by forming the light emitting elements and the photoelectric conversion elements on the same substrate.
The light emitting elements function as light sources, and light emitted from the light emitting elements is reflected by a subject, and then irradiated to the photoelectric conversion elements. An electric current occurs at this point when the light reflected by the subject is irradiated to the photoelectric conversion elements, and an electric signal with image information of the subject (image signal) is taken into the semiconductor device. Image information can thus be read by using the photoelectric conversion elements. The light emitted from the light emitting elements is uniformly irradiated to the subject with the above structure, and therefore irregularities of the read-in image in brightness do not develop in the semiconductor device of the present invention.
Further, a signal line driver circuit and an output switching circuit are used as driver circuits of the semiconductor device in the present invention. The signal line driver circuit outputs timing signals to the output switching circuit based on signals input from the outside. The output switching circuit outputs different timing signals to signal lines connected to the TFTs of the light emitting element portion, and to signal lines connected to the TFTs of the sensor portion. In other words, it becomes possible with employing the output switching circuit to control two signal lines by using one driver circuit. As a result, it becomes possible to make the area occupied by the driver circuits of the semiconductor device smaller, and a reduction in size of the semiconductor device can be realized.
Note that the present invention is effective in semiconductor devices with any kind of structure which has light emitting elements and photoelectric conversion elements. Furthermore, the present invention is also effective in semiconductor devices which has liquid crystal elements that use a front light or a back light as a light source, instead of light emitting elements.
Note that the term xe2x80x9cconnectionxe2x80x9d has a meaning of electrical connection throughout this specification.